Hybrid electric vehicles offer potential improvements in fuel economy and further reductions in emissions. A hybrid electric vehicle incorporates a traditional internal combustion engine combined with an electromechanical transmission having one or more electric motor/generators arranged in series or parallel and gearing arrangements such as planetary gear sets. The electric motor/generators assist in propulsion or energy absorption (storage) depending on the mode of operation. As with any energy conversion device, the motor/generators are less than 100 percent efficient, and reject some energy as heat. Efficient removal of this waste heat is required in order to achieve the objective of a fuel-efficient vehicle.